1 Field of the Invention
The present invention relates to a method of finishing an edge of a sheet glass for use in e.g. fire-resistant construction material such as a fire-resistant door, a fire-resistant window or the like, a heat-tempered sheet glass using the method, and relates also to a fire-resistant construction material using this heat-tempered sheet glass.
2 Description of the Related Art
When a sheet glass is used in a fire-resistant construction material such as a fire-resistant door, it is necessary for the glass to have a high edge strength in order to avoid a heat cracking phenomenon of this sheet glass. Namely, when a sheet glass is subjected to a significant heat, heat expansion occurring at the center area of the sheet glass applies a tension to the edge of the sheet glass fixedly supported to a window sash, for example. Then, when this tension exceeds the edge strength of the sheet glass, a cracking occurs in the sheet glass. This is the heat cracking phenomenon.
As fire-resistant sheet glass, there are known a wired sheet glass and a laminated glass incorporating an intermediate layer made of silica hydrate alkaline. In the case of the former, its transparency may be a problem since the embedded wire hinders the the view. The latter also has the possibility of losing its transparency due to bubbling of the intermediate layer resulting from a thermal variation. As an alternative free from such drawbacks, there is known a sheet glass prepared by heat-tempering treatment of soda-lime glass material.
As such heat tempering treatment, the convention has provided a method comprising the steps of: heating a sheet glass at a temperature range (approximately, 760 deg. C.) significantly higher than a melting point of the glass (720 to 730 deg. C.); and then spraying a cooling air against the sheet glass at an extremely high pressure such as a back pressure of 950 mmAq, for instance. With such heat tempering treatment of the sheet glass as above, it is possible indeed to provide the sheet glass with a predetermined edge strength. However, since the air is sprayed at the very high pressure against the sheet glass which has been heated up to such high temperature range as exceeding the melting point of the glass, this spraying may result in surface irregularity or warping of the sheet glass which leads to distortion of the image reflected by the sheet glass.
According to a conventional method of finishing an edge of such sheet glass as described supra, in order to obtain a flat and even edge, as illustrated in FIGS. 6(a) and (b), a cup abrading wheel 20 is rotated about an axis while causing a side face 20a of the wheel to contact an edge 21a or a ridge 21b of the sheet glass 21. The cup abrading wheel includes diamond or abrading stone or the like affixed on the side face 20a thereof.
With such sheet glass edge abrading method as described supra, a very fine abrade streak, which is hardly recognizable with naked eyes, is formed at the ridge portion as a result from the abrading of the edge portion. For this reason, a thermal stress associated with a thermal expansion tends to be concentrated at the abrade streak. In particular, in the case of the soda-lime glass (without heat-tempering treatment), it is not possible to provide this glass with a high edge strength.
Accordingly, in order to allow its use as a fire-resistant sheet glass, the above-described heat tempering treatment is needed. But, this treatment involves the inconveniences described supra. Further, if the heat tempering treatment is effected at a lower heating temperature and/or lower back pressure of the spraying air than the conventional method in order to avoid the inconveniences (surface irregularity, warping) of the heat tempering treatment, then, this will result in an insufficient edge strength of the sheet glass obtained as a fire-resistant door.
In view of the above-described state of the art, the primary object of the invention is to solve the drawbacks of the conventional art by providing an improved method of finishing an edge of a sheet glass capable of increasing the edge strength of the glass, a heat-tempered glass with a predetermined edge strength made by using the method, and also a fire-resistant construction material using this heat-tempered sheet glass.
For fulfilling the above-noted object, a method of finishing an edge of a sheet glass, according to the present invention, comprises:
a first grinding step for abrading an edge of a sheet glass so as to provide the edge with an outwardly convex curved shape; and
a finishing step for finishing a bordering portion which has been formed by the first abrading step between the curved edge and each flat side face of the sheet glass, so as to provide this bordering portion with a greater smoothness than smoothness provided by the first abrading step.
In general, the stress occurring within a sheet glass is apt to be concentrated at the ridge portions of the sheet glass. Then, according to the sheet glass edge finishing method of the present invention, the method comprises the first abrading step for abrading an edge of a sheet glass so as to provide the edge with an outwardly convex curved shape; and a finishing step for finishing a bordering portion which has been formed by the first abrading step between the curved edge and each flat side face of the sheet glass, so as to provide this bordering portion with a greater smoothness than smoothness provided by the first abrading step. Hence, it is possible to restrict concentration of stress at the edge portion of the sheet glass. Especially, by effecting the finishing step for finishing a bordering portion which has been formed by the first grinding step between the curved edge and each flat side face of the sheet glass, so as to provide this bordering portion with a greater smoothness than smoothness provided by the first abrading step, the ridge portions are substantially eliminated to provide a smoother finished surface. Accordingly, it is possible to avoid the formation of the abrade streak which tends to invite the concentration of the stress at this portion. As a result, the method of the invention allows the sheet glass to obtain a higher edge strength than the convention.
With the increased edge strength, even if the heat tempering operation is effected at a lower temperature range and/or lower air-spraying pressure than the convention, the resulting sheet glass may retain a predetermined sufficient edge strength after the heat-tempering treatment. As a result, it becomes possible to avoid the inconveniences of the prior art such as the surface irregularity or warping which leads to a reflection image distortion. Moreover, the running costs of the heat-tempering system may be reduced as well.
In summary, according to the invention""s method of finishing the edge of a sheet glass, it becomes possible to restrict concentration of thermal stress at the edge of the sheet glass and hence to increase the edge strength of the sheet glass. As the result, even if the heat-tempering treatment of the sheet glass is effected by a simpler method than the conventional method, the resulting glass may maintain sufficient performance as a fire-resistant sheet glass. Accordingly, the method achieves improvement of the quality of the sheet glass and also reduction of the running cost of the heat-tempering treatment system.
The finishing step may be carried out by a variety of manners such as buffing, heat melting, chemical melting or the like.
If the finishing step is carried out by buffing, it is possible to polish the surface with the irregularity in the order of a few um""s.
Further, if the finishing step is carried out by heat melting, it is possible to provide the finished surface with the same finish as the sheet glass per se, so that the stress applied to the sheet glass may be born by the entire edge thereof. As a result, the edge strength of the sheet glass may be further increased.
If the finishing step is carried out by chemical melting, this finishing step per se may be carried out by a simple process, so that the efficiency of the edge finishing operation of the sheet glass may be improved.
For accomplishing the above-noted object, in a heat-tempered sheet glass, according to the present invention, the sheet glass has its entire surface subjected to a heat-tempering treatment, the sheet glass comprises an edge thereof formed as an outwardly convex curved edge having a intermediate portion in the thickness direction thereof projecting outwards in the direction of the plane of the sheet glass;
wherein, the curved edge has a maximum surface irregularity ranging below 0.05 mm; and
a bordering portion formed between the curved edge and each flat side face of the sheet glass has a maximum surface irregularity ranging below 0.007 mm.
With the above construction, the edge of the sheet glass having its entire face thereof subjected to a heat-tempering treatment is formed as an outwardly convex curved edge portion having an intermediate portion in the direction of thickness thereof projecting outwards in the direction of the plane of the sheet glass, and this curved edge portion has a maximum surface irregularity ranging below 0.05 mm. Further, a bordering portion formed between the curved edge portion and a flat portion on each side face of the sheet glass has a maximum surface irregularity ranging below 0.007 mm. Accordingly, this heat-tempered glass may effectively restrict concentration of stress at the edge portion.
More particularly, the curved edge portion formed at the edge of the sheet glass is provided with very smooth finish of maximum surface irregularity ranging lower than 0.05 mm. Moreover, no ridges are formed at this curved edge portion. Hence, concentration of stress at the curved edge portion may be avoided. Further, the bordering portion formed between the curved edge portion and a flat portion on each side face of the sheet glass is provided with even smoother finish having a maximum surface irregularity ranging below 0.007 mm. Therefore, the entire sheet glass too may be free from ridges which tend to invite concentration of stress. As a result, it becomes possible to avoid the stress concentration at the edges of the sheet glass, whereby the edge strength of this sheet glass may be increased to be higher than 4 kgf/mm2 approximately.
Incidentally, in the case also of a curved shape which is structurally advantageous for avoiding stress concentration, if its surface irregularity ranges over 0.05 mm, the stress concentration still tends to occur at the ridges and bottoms of this uneven surface. Further, in the case of the bordering portion between the curved edge portion and each flat side face of the sheet glass, as this portion comprises a xe2x80x98inter-face transition regionxe2x80x99, the stress concentration tends to occur at the ridges and bottoms of its irregularity if the irregularity is greater, at its maximum, than 0.007 mm.
With the increased edge strength described above, even if the heat-tempering operation is effected by a heating operation at a temperature range lower than that of the convention or by an air spraying operation at an air pressure lower than that of the convention, the resultant glass may still be free from such inconvenience of the conventional art as the surface irregularity of warping which distorts the reflection image while maintaining a predetermined sufficient edge strength.
Therefore, with the heat-tempered glass according to the present invention, even if the heat tempering operation of the sheet glass is effected by a simpler method than those employed in the convention, the resultant sheet glass may still maintain good performance as a heat-resistant glass. So that, it is possible to improve the quality of the sheet glass and also to reduce the running costs of the system used for the heat tempering operation.
Further, as described in the appended claim 5, preferably, the surface compressing stress used in the heat tempering treatment ranges over 17 kgf/mm2.
Incidentally, if a sheet glass is to be used as a Class A or Class B fire-resistant door according to a fire resistant testing based on the Japanese Construction Ministry Public Notice No. 1125, there is the possibility of danger of heat cracking in the sheet glass unless the sheet glass has an edge strength higher than 21 kgf/mm2 (in case the sheet glass is supported by a retainer metal element 9 having good heat conduction characteristics, with an engaging depth d at a peripheral edge of the sheet glass with a sash being 10 mm approximately, as illustrated in FIG. 2). In this respect, with the construction set forth in claim 5 of the present invention, the surface compressing stress used in the heat tempering treatment is provided to be higher than 17 kgf/mm2. Then, in combination with the edge strength higher than 4 kgf/mm2 obtained by the edge finishing process described supra, the sheet glass may obtain an edge strength which is in effect higher than 21 kgf/mm2. Accordingly, this sheet glass may be used as the Class A or B fire-resistant door without any problems.
Incidentally, if the sheet glass is supported with a greater engaging depth of about 15 mm, there occurs a slightly greater temperature difference between the peripheral edge and the center portion of the sheet glass. Therefore, a greater surface compressing force greater than 18 kgf/mm2 will be needed in this case.
Further, if the sheet glass is supported with the greater engaging depth of about 15 mm and without the use of the retainer metal element 9 (see FIG. 5), the surface compressing stress needs to be still higher than 22 kgf/mm2.
As described above, in any of the various support conditions, the sheet glass may be used as the Class A or B fire-resistant door.
A fire-resistant construction material according to the present invention is characterized in that the material uses the heat-tempered sheet glass described above. Then, this fire-resistant construction material may be used as a fire-resistant door or fire-resistant window having superior fire resistance.
Further and other objects, features and effects of the invention will become more apparent from the following more detailed description of the embodiments of the invention with reference to the accompanying drawings.